ZnO, which has been used for varistors, gas sensors, sun block materials and the like, has recently been a target of attention as being applied to light emitting elements, piezoelectric elements, transparent electrodes and the like owing to optical characteristics and piezoelectric characteristics thereof. Especially, research and development activities are now eagerly conducted to apply ZnO for semiconductors used for light emitting elements which emit light of a short wavelength in the blue to ultraviolet range and applications thereof since ZnO has a direct bandgap of 3.3 to 3.4 eV like GaN. In order to apply ZnO for these uses and develop such uses, it is important to establish a method for producing high quality ZnO.
Conventional methods for growing ZnO, especially a ZnO single crystal, are roughly classified into gas phase growth techniques and liquid phase growth techniques. As the gas phase growth techniques, chemical vapor transport (see Japanese Patent Application Laid-Open No. 2004-131301), molecular beam epitaxy, metal organic chemical vapor deposition (see Japanese Patent Application Laid-Open No. 2004-84001), sublimation (see Japanese Patent Application Laid-Open No. 5-70286) and the like are used. These techniques cause many dislocations, defects and the like, and the quality of the resultant crystals is insufficient.
A method of producing a single crystal thin film of an oxide or a fluoride on a substrate by a gas phase growth technique is disclosed (see Japanese Patent Application Laid-Open No. 4-367588). This publication discloses “a method for epitaxially growing a single crystal of any compound of an oxide or a fluoride on a crystalline substrate, characterized by keeping a solution, obtained by dissolving the compound in a melt of a melting agent, in a crucible under an atmospheric pressure, keeping it at a temperature at which the solution is vaporized but the components of the solvent are not vaporized, solidifying a vapor of the compound vaporized from the solution on a surface of the crystalline substrate held on the solution and kept at a temperature lower than that of the solution, thereby forming a single crystal of the compound; and a method for epitaxially growing zinc oxide characterized in that the melting agent contains one or at least two of lead monoxide, lead fluoride, boron oxide and vanadium oxide as a main substance”.
In the section of the examples, an example in which zinc oxide is used as a crystalline component, boron oxide and lead monoxide are used as melting agents, and only zinc oxide is vaporized steadily to epitaxially grow zinc oxide on a sapphire substrate. However, this method uses a gas phase growth technique and so involves a problem that the quality of the crystal is low.
A liquid phase growth technique, by which crystal growth proceeds with thermal equilibrium on principle, has an advantage of being capable of producing high quality crystals more easily than a gas phase growth technique. However, ZnO has a melting point as high as about 1975° C. and easily vaporizes and so it is difficult to grow ZnO by the Czochralski technique, which is used with silicon single crystal or the like. Therefore, a slow cooling technique, a hydrothermal technique, a flux technique, a floating zone technique, a TSSG technique, a solution growth technique and the like are used, by which a target subject is dissolved in an appropriate solvent and the temperature of the obtained mixture solution is decreased to realize a supersaturation state, thereby growing the target substance from the melted solution.
For growing a ZnO single crystal by a solution growth technique, a solvent capable of dissolving ZnO is needed. Substances used as the solvent are PbF2, PbO, high temperature-high pressure water, V2O5, B2O3, MoO3 and the like. Hereinafter, the problems of these solvents will be described.
A method of growing a ZnO single crystal by a slow cooling technique with PbF2 as the solvent is disclosed (see J. W. Nielson and E. F. Dearborn, J. Phys. Chem. 64, (1960) 1762). FIG. 1 shows a PbF2—ZnO phase diagram included in this document. PbF2 and ZnO form an eutectic system. The composition of the eutectic contains ZnO at 8.8%, and the temperature thereof at this point is about 730° C. At or in the vicinity of a ZnO concentration of 8.8 mol %, PbO is also deposited. Therefore, the ZnO concentration needs to be at least about 10 mol %. In order to dissolve 10 mol % or more of ZnO, the temperature needs to be 770° C. or higher. A general practice to establish a stable and well-reproduceable method for growing a single crystal is to keep the solution at a temperature higher by 100 to 200° C. than the melting point thereof such that the solvent and the solute are uniformly mixed together. When the PbF2 as the solvent is kept at about 870 to 970° C., PbF2 is partially vaporized and so the composition with ZnO is varied. For this reason, it is difficult to stably grow a ZnO single crystal. In addition, since the vaporized PbF2 reacts with materials of the members of the furnace, the number of times that the members is usable is reduced and also hazardous Pb compounds volatize. This, for example, requires the growth furnace to have a sealable structure, which increases the production cost.
PbO as the solvent has a disadvantage of having a high vapor pressure like PbF2. FIG. 2 shows a PbO—ZnO phase diagram (see M. P. Bauleke, K. O. McDowell, J. Am. Ceram. Soc. 46[5] 243 (1963)). In order to dissolve about 12 mol % or more of ZnO, which is the only substance deposited, the temperature needs to be 861° C. or higher. In order to establish a stable and well-reproduceable method for growing a single crystal, the solution needs to be kept at about 960° C. to 1060° C. such that the solvent and the solute are uniformly mixed together. When the PbO as the solvent is kept at about 960 to 1060° C., PbO is partially vaporized and so the composition with ZnO is varied. For this reason, it is difficult to stably grow a ZnO single crystal. In addition, since the vaporized PbO reacts with materials of the members of the furnace, the number of times that the members is usable is reduced and also hazardous Pb compounds volatize. This, for example, requires the growth furnace to have a sealable structure, which increases the production cost.
With a hydrothermal technique using high temperature-high pressure water as the solvent, a relatively high quality ZnO single crystal is obtained. However, this technique requires about 2 weeks to obtain a crystal of about 10 mm cube and so has a problem of a slow growth rate (Sekiguchi, Miyashita, et al., Journal of the Crystallographic Society of Japan, 26(4) (1999) 39). A method of growing a ZnO single crystal by a solution growth technique or a traveling solvent zone melting technique using V2O5 and/or B2O3, MoO3 as the solvent is disclosed (see Japanese Laid-Open Patent Publications Nos. 2002-193698 and 2003-2790, K. Oka and H. Shibata, J. Cryst. Growth 237-239 (2002) 509). Such a method can grow a ZnO single crystal on a seed crystal or substrate, but the resultant crystal is colored and is not considered to have a high level of crystallinity.
As described above, it is difficult to grow a ZnO single crystal stably and at low cost using PbF2 or PbO as the solvent because the vapor pressure is too high. A hydrothermal technique using high temperature-high pressure water requires a period as long as 2 weeks to obtain a crystal of about 10 mm cube. A solution growth technique or a traveling solvent zone melting technique using V2O5 and/or B2O3, MoO3 as the solvent has a problem that the quality of the crystal is low.
Use of PbF2 as the solvent has a problem that the grown ZnO single crystal is contaminated with many fluorine (F) atoms mixed therein. Especially, the electric properties of semiconductor crystals are sensitive to the structure thereof, and impurity is one type of disturbance to the structure. A semiconductor is useful because the electric properties significantly vary in accordance with the type and concentration of the impurity added, and therefore it is very important to control the impurity. ZnO can be provided with n-type conductivity by substituting the O atom with a group VII atom such as fluorine or the like or by substituting the Zn atom with a group III atom such as boron, aluminum, gallium, indium or the like. ZnO can be provided with p-type conductivity by substituting the O atom with a group V atom such as nitrogen or the like or by substituting the Zn atom with a group I atom such as lithium or the like. However, the above-described method causes the grown single crystal to be contaminated with many fluorine atoms, which provide ZnO with n-type conductivity but are difficult to be controlled or inhibit ZnO from being provided with p-type conductivity.
There is another problem that the ZnO thin film is contaminated with many Al impurities from the materials of the members of the production furnace. When a conventional production furnace is used, the grown single crystal is contaminated with many Al impurities, which provide ZnO with n-type conductivity but are difficult to be controlled or inhibit ZnO from being provided with p-type conductivity.